1. Field of the Invention
The invention relates generally to adsorbent compositions, and in particular to adsorbent compositions which are selective for para-xylene and are useful for vapor-phase adsorption processes.
2. Brief Description of Related Technology
Hydrocarbon mixtures or fractions containing C8+ aromatics are often by-products of certain oil refinery processes including, but not limited to, catalytic reforming processes. These hydrocarbon mixtures typically contain up to about 30 weight percent (wt. %) C9+ aromatics, up to about 10 wt. % non-aromatics, up to about 50 wt. % ethylbenzene, with the balance (e.g., up to about 100 wt. %) being a mixture of xylene isomers. Most commonly present among the C8 aromatics are ethylbenzene (“EB”), and xylene isomers, including meta-xylene (“mX”), ortho-xylene (“oX”), and para-xylene (“pX”). Typically, when present among the C8 aromatics, ethylbenzene is present in a concentration of up to about 20 wt. % based on the total weight of the C8 aromatics. The three xylene isomers typically comprise the remainder of the C8 aromatics, and are typically present at an equilibrium weight ratio of about 1:2:1 (oX:mX:pX, respectively). Thus, as used herein, the term “equilibrated mixture of xylene isomers” refers to a mixture containing the isomers in the weight ratio of about 1:2:1 (oX:mX:pX).
Efficient separation of the C8 aromatics fraction into its individual constituents is of interest because the individual, isolated C8 aromatic constituents are useful commodity chemicals. For example, ethylbenzene is useful in making styrenes; meta-xylene is useful in making isophthalic acids; ortho-xylene is useful in making phthalic anhydrides; and, para-xylene is useful for making terephthalic acids, which are useful for the synthesis of many commercially important resins, including polyesters such as polybutene terephthalate, polyethylene terephthalate, and polypropylene terephthalate.
However, simply separating the C8 aromatics made available from a particular source may not provide sufficient quantities of a desired C8 aromatic constituent to meet market requirements. For example, it is generally desirable to increase or maximize para-xylene production from a particular C8 aromatic mixture because there is generally a higher demand for para-xylene when compared with the other C8 aromatic constituents. Thus, separation of para-xylene is often coupled with isomerization of the remaining stream containing the meta- and ortho-xylene isomers to convert a portion of the meta- and ortho-xylene isomers to the desired para-xylene by reestablishing the equilibrium between the xylene isomers. Conventional C8 aromatic separation methods may further include a method for the conversion of ethylbenzene into benzene, which can be more easily separated from the xylene isomers by distillation.
Typically, such a hydrocarbon mixture is passed through a separation or fractionation column to remove higher boiling C9+ hydrocarbons. A second, lower-boiling fraction can be, removed from a preceding or subsequent column so that the remaining fraction is predominantly a C8 aromatic hydrocarbon mixture.
In general, para-xylene is recovered by subjecting the C8 aromatic mixture to one or more separation steps. Performing fractional distillation on the C8 aromatic mixture is impractical because ethylbenzene, meta-xylene, ortho-xylene, and para-xylene have similar boiling points (falling between about 136° C. and about 144° C.). Thus, separation of para-xylene is generally done by crystallization and/or liquid-phase adsorption chromatography.
Crystallization processes exploit the differences in freezing or crystallization temperatures of the various xylenes—para-xylene crystallizes at about 13.3° C. while ortho-xylene and meta-xylene crystallize at about −25.2° C. and about −47.9° C., respectively. In the physical system of the three xylene isomers, there are two binary eutectics of importance: the para-xylene/meta-xylene binary eutectic and the para-xylene/ortho-xylene binary eutectic. As para-xylene crystallizes from the mixture, the remaining mixture approaches one of these binary eutectics, depending upon the starting composition of the mixture. Therefore, in commercial-scale processes, para-xylene is crystallized such that the binary eutectics are approached—but not reached—to avoid co-crystallization of the other xylene isomers, which would lower the purity of the obtained para-xylene. Because of these binary eutectics, the amount of para-xylene recoverable per pass through a crystallization process is generally no greater than about 65% of the amount of para-xylene present in the stream fed to the crystallization unit. Furthermore, crystallization can be very expensive because the various xylene isomers crystallize at very low temperatures.
U.S. Pat. No. 5,329,060 to Swift (Jul. 12, 1994) discloses increasing recovery of para-xylene by separating the xylene isomers in an adsorption step prior to crystallization. Swift discloses that liquid phase adsorption is preferred because of the reduced temperature requirements and decreased opportunities for side reaction. In the adsorption step, a crystalline zeolitic aluminosilicate adsorbent having a silicon to aluminum ratio between about two and about six selectively adsorbs meta-xylene and ortho-xylene (or alternatively para-xylene) to provide a para-xylene-enriched stream, which is subsequently directed to a crystallization apparatus. The para-xylene lean stream is generally pressurized and reacted in the presence of an isomerization catalyst to obtain an equilibrated mixture of xylene isomers, which can then be recycled to the liquid adsorber.
Liquid-phase adsorption chromatography refers to chromatographic processes in which a mixture comes into contact with a stationary phase and a liquid mobile phase. Separation of the mixture components occurs because of the differences in affinity of the components for the stationary and mobile phases of the chromatographic system. Liquid-phase adsorption chromatographic separations are typically batch processes. Simulated moving bed adsorption chromatography (SiMBAC) is a continuous operation that utilizes the same principles to achieve separation. SiMBAC, however, has its limits and is expensive to operate because it requires an internal recycle of large volume(s) of various hydrocarbon desorbent material(s). Additionally, the effluent streams from the adsorption step must be separated from the desirable products in downstream distillation steps. Thus, conventional liquid-phase adsorption chromatographic processes are disadvantageous because of significant capital and energy costs.
It has been found that the foregoing crystallization and SiMBAC steps can be made more attractive if the feedstock(s) for those steps were reformulated to contain a higher-than-equilibrium concentration of para-xylene. Higher-than-equilibrium concentrations of para-xylene may be obtained by selective toluene disproportionation as described in, for example, International (PCT) Publication Nos. WO 00/69796 (Nov. 23, 2000) and WO 93/17987 (Sep. 16, 1993).
Additionally, crystallization and SiMBAC steps can be designed and operated to concentrate para-xylene streams for subsequent purification steps. However, such concentration steps typically suffer from many (or all) of the disadvantages previously discussed with respect to crystallization and SiMBAC processes.
Feedstocks with higher-than-equilibrium para-xylene concentrations may also be produced by vapor-phase adsorption processes, including pressure swing adsorption (“PSA”) processes. PSA processes have been widely practiced for the separation of gases, such as, air into nitrogen and oxygen, removal of water from air, and hydrogen purification, and are generally described in Ralph T. Yang, “Gas Separation by Adsorption Processes,” pp. 237–274 (Butterworth Publishers, Boston, 1987) (TP242.Y36). PSA processes generally use a solid adsorbent that preferentially adsorbs some components from a mixture over other components in the mixture. Typically, the total pressure of the system is reduced to recover the adsorbate. In contrast, partial pressure swing adsorption (PPSA) operates at a substantially non-decreasing pressure and uses an inert gas, such as hydrogen or nitrogen, to purge or sweep the adsorbate from the adsorbent. Thus, PPSA processes are based on swings in “partial” pressure, rather than lowering the total unit pressure, as is traditionally practiced with PSA processes. Thermal swing adsorption (TSA) processes are gas-phase adsorption processes, wherein the adsorbate is recovered by raising the temperature of the adsorbent bed. Typically, adsorbate recovery is accomplished by purging the bed with a preheated gas.
Chinese Patent Publication No. 1,136,549 A (Nov. 27, 1996) to Long et al. discloses a method of producing para-xylene by passing a gaseous C8 hydrocarbon mixture through an adsorption bed containing an adsorbent that is selective for para-xylene and ethylbenzene to obtain, after suitable desorption, a stream containing meta-and ortho-xylenes and a stream containing para-xylene and ethylbenzene. The adsorption is carried out at a temperature between 140° C. and 370° C., a pressure between atmospheric pressure and 300 kilopascals (kPa), and using a Mobil-5 (MFI) type molecular sieve adsorbent, including ZSM-5 (available from ExxonMobil Chemicals), ferrierite, and silicalite-1 zeolite molecular sieves, and in particular, binder-free silicalite-1 zeolite molecular sieves. The desorption of the para-xylene and ethylbenzene can be carried out with an aqueous vapor desorbent at a temperature within the same range of the adsorption, and at a pressure between atmospheric and 1000 kPa. Alternatively, the desorption can be carried out without a desorbent, and accomplished by mere decompression at a pressure between 1 kPa and 4 kPa.